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The present invention relates to MR patient support configurations and more specifically to a patient support that mounts to a main MR magnet and supports a patient table bridge.
An MR imaging system or scanner commonly includes a cryostat, which contains a powerful superconductive main magnet positioned around a main magnet bore. The superconductive magnet is maintained at an extremely cold temperature and produces a strong static magnetic field Bo along a bore axis within the main magnet. Other essential components of the MR system include an RF coil, or RF antenna, and a gradient coil assembly, which comprises a hollow cylindrical structure. The RF coil may be operated in a transmit mode, to generate MR signals in an imaging subject, or may be operated in a receive mode to detect the MR signals.
The gradient coils are electrically excited to impose X-, Y-, and Z-gradient time varying magnetic fields on the primary magnetic fields that are required for imaging purposes. In a common arrangement, each gradient field is produced by a pair or set of gradient coils, wherein each coil is wrapped around one of two cylindrical coil forms. The two coil forms are placed in coaxial relationship, and the coil forms and respective X-, Y- and Z-gradient coils collectively comprise a gradient coil assembly. Arrangements of this type are described, for example, in U.S. Pat. No. 5,570,021, issued Oct. 29, 1996 and commonly assigned herewith to the General Electric Company. Such arrangements are also described in U.S. Pat. No. 5,760,584, issued Jun. 2, 1998 and likewise assigned to the General Electric Company. Typically, the gradient coil assembly is mechanically supported within the cylindrical bore of the main magnet. The gradient coil, RF coil and main magnet together form an imaging area about the main magnet bore axis.
To support a patient within the main magnet bore during data acquisition, a patient support structure is provided that typically includes an upright support, a bridge, a cradle and a bridge support. The upright support is a rigid member that rests on a floor adjacent an MR imaging system within an imaging room and includes an upper end for receiving a first end of the bridge. The bridge includes a stiff member that extends between first and second ends where the first end is mounted to the rigid upright support and the second end extends into and through the imaging area so that its length is generally parallel to the bore axis. In many designs the bridge includes tracks for receiving cradle wheels and guiding the cradle through the imaging area.
The cradle is typically a relatively flimsy member having upper and lower oppositely facing surfaces and a length that is generally sufficient to support a patient. The cradle includes a plurality of wheels mounted to its lower surface and arranged so as to be receivable within the bridge tracks for guidance there along. The cradle is capable of movement along the bridge into various positions with respect to the imaging area including a loading position outside the imaging area and at least one imaging position where at least a portion of a patient disposed on the cradle is positioned within the imaging area.
As configured above, when a patient (especially a relatively heavy patient) is supported on the cradle and the cradle is fully extended on the bridge, the bridge has a tendency to deflect slightly downward thereby causing a patient misalignment. To overcome this problem many support configurations include a bridge support. To this end, an exemplary bridge support includes a rigid member that is typically mounted to the inside of the gradient coil and extends upwardly to and is secured to the bridge relatively closer to the second end of the bridge than the first end. Thus, when a patient is positioned within the imaging area, the cradle and bridge are supported by the upright support on the first end and by the bridge support, gradient coil and main magnet on the second end.
Unfortunately, when a gradient coil is excited to generate magnetic gradients, the gradients interact with structure about the coils and the coils tend to be mechanically displaced (i.e., vibrates). The mechanical structure used to support the gradient coil assembly within the main magnet bore provides a path for transferring or coupling the vibrations of the gradient coils to the main magnet structure. Generally, the main magnet is supported on the floor of the building in which the MR system is operated. Accordingly, the gradient generated vibrations are often directly coupled from the magnet to the floor, and then travel through the floor to vibrate structures throughout the building. As a result, gradient coil vibrations can couple acoustically to rooms outside of the MR scan room, i.e., a room which is specially constructed to house the MR system.
In addition to the problems associated with transmitting gradient vibrations to other facility equipment and space, the rigid bridge support also causes the gradient vibrations to be transmitted to the patient support cradle and a patient thereon. While gradient related patient vibration in early MR systems was relatively minimal and therefore could essentially be ignored, characteristics of newer MR systems have resulted in greater adverse effects. For instance, gradient technology has evolved to the point where relatively high gradient fields are employed during data generation and acquisition so that the magnitude of gradient related vibrations is relatively greater in newer systems. In addition, the actual mass of the imaging components (i.e., the main magnet, coils, shields, etc.) has been reduced appreciably such that even small gradient fields sometimes cause appreciable vibration.
Cradle vibration has two adverse side effects. First, whenever a patient is exposed to a new or unfamiliar medical process, the patient typically and understandably experiences anxiety and nervousness about the experience. This is especially true of MR imaging procedures where noise and essentially uncontrolled gradient movement and vibrations are transmitted to the patient. Anxiety often causes patients to move or flinch during acquisition which can cause image artifacts in images generated with collected data. Second, even where a patient manages to remain essentially still relative to a supporting cradle, where the cradle and patient vibrate together relative to the RF data receiving coils, resulting images are typically polluted by image artifacts.
One configuration that essentially isolates the gradient coils from the main magnet and thereby mitigates transmission of gradient vibrations to the main magnet and surrounding facility equipment and space is described in U.S. Pat. No. 6,160,399 (hereinafter xe2x80x9cthe ""399 patentxe2x80x9d) which issued on Dec. 12, 2000 and is entitled xe2x80x9cApparatus For Supporting MR Gradient Coil Assemblyxe2x80x9d. According to the ""399 patent, two mounting assemblies are mounted to the main magnet and extend axially to the gradient coils, a separate mounting assembly disposed at either end of the main magnet. The mounting assemblies transmit a static force from the main magnet to the gradient coil assembly to hold the coil assembly in place within the main magnet bore in coaxial relationship with the bore and in spaced-apart relationship with an internal bore wall. At the same time, the two mounting assemblies act to dampen or attenuate the gradient coil vibrations, and thus oppose passage transmission of the vibrations through the mounting assemblies to the main magnet. The ""399 patent configuration provides no other path through which gradient coil vibrations can be transferred from the gradient coil assembly to the main magnet.
While addressing the problem of transmitting gradient vibrations from the MR configuration to facility equipment and space, unfortunately the mounting assemblies described in the ""399 patent do nothing to reduce vibrations to the patient support cradle. In fact, in some cases, by isolating the gradient coil from the main magnet, the end result may be to increase cradle and patient vibrations thereby increasing discomfort and reducing image quality.
It has been recognized that a patient support table or cradle can be sufficiently isolated from MR gradient coils by, instead of supporting the cradle on the coils, providing a bracket that mounts directly to the MR main magnet and supports the cradle while in an imaging area. This concept is particularly useful where the coil gradient assembly is isolated from the main magnet as in the case of the ""399 patent referenced above as the combined coil isolation system of the ""399 patent and the present invention increase patient comfort and reduce imaging artifacts appreciably.
An exemplary embodiment of the invention is to be used with an MR system having a main magnet, a gradient coil assembly and a patient support, the magnet having first and second oppositely facing surfaces and forming a bore that extends between the first and second surfaces along a bore axis, the coil assembly disposed within the bore about an imaging area, the support including a support member and a bridge having first and second ends, the support member supporting the first bridge end proximate the first surface, the bridge extending into and through the bore so that the second end is proximate the second surface and the bridge forms an essentially downwardly facing undersurface. The apparatus is for supporting the bridge and comprises a bracket disposed for fixable attachment to the main magnet such that the bracket extends toward the imaging area. The bracket forms at least one essentially upwardly facing support surface for receiving the bridge undersurface and supporting the bridge there above. In several embodiments the bracket is mountable to the second magnet surface.
In one embodiment the bracket includes first and second post members that extend essentially upwardly to distal ends, the distal ends forming first and second upwardly facing surfaces for supporting the bridge, respectively. Each post may include a stainless steel rod having a distal end and a phenolic head member.
In some embodiments the bridge has a width dimension perpendicular to the bore axis, the bracket has a length dimension greater than the width dimension and the bracket is securable to the main magnet at opposite ends of the length dimension so as to have an essentially horizontal orientation.
In most embodiments the bracket components are formed of a low magnetic flux material such as, for instance, a phenolic resin material.
These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.